#include<iostream>
using namespace std;
#include<vector>
class Solution {
public:
    int singleNonDuplicate(vector<int>& nums) {
        int n = nums.size();
        int left = 0, right = n - 1;
        while (left < right) {
            int mid = left + (right - left) / 2;
            if (nums[mid] == nums[mid ^ 1])
                left = mid + 1;
            else
                right = mid;
        }
        return nums[left];
    }
};